Lattice phonon modes of the spin crossover crystal [Fe(phen)2(NCS)2] studied by THz, IR, Raman spectroscopies and DFT calculations

Abstract

Fe(phen)2(NCS)2] is a prototype transition metal complex material, which undergoes a phase transition between low-spin (LS) and high-spin (HS) phases, induced by temperature, pressure or light. Vibrational modes play a key role for spin-state switching both in thermal and photo-induced cases, by contributing to vibrational entropy for thermal equilibrium transitions or driving the fast structural trapping of the photoinduced high spin state.Here we study the crystal phonon modes of [Fe(phen)2(NCS)2], by combining THz, IR, and Raman spectroscopies sensitive to modes in different frequency ranges and different symmetries. We compare the experimental results to DFT calculations performed in a periodic 3D crystal for understanding the phonon modes in the crystal, compared to molecular vibrations. Indeed, each vibrational mode of the isolated molecule combines into several modes of different symmetry and frequency in the crystal, as the unit cell contains four molecules. We focus our attention on the HS symmetric and anti-symmetric breathing modes in the crystal as well as on the N-CS stretching modes.

Vibrational properties and cooperativity of the 3D spin crossover network [Fe(pyrazine)][Pt(CN)4]

“…In order to obtain the pDOS of the 3D network of covalent bonds of the system under study in ideal case solid-state DFT frequency calculations could be performed. However, in spite of recently reported results 15 there still is no established solid DFT procedure allowing the calculations of both acoustic and optical phonons with reliable exchange-correlation functionals and basis sets. Therefore, we decided to use large model molecules of 1 in their HS and LS states.…”

“…Calculated and experimental15 bond lengths for 1. The calculated Pt-CN distances correspond to the Pd-CN distances as used in the model molecule.…”

Relationship between the Molecular Structure and Switching Temperature in a Library of Spin-Crossover Molecular Materials

“…The chemical pressure exerted on a molecule by its nearest neighbors constrains its geometry, changing the relative enthalpies of the spin states as the transition proceeds . Phonon entropy also contributes to SCO thermodynamics in condensed phases, as a function of the lattice symmetry as well as the intra- and intermolecular bonding interactions …”

Exploring the light-induced dynamics in solvated metallogrid complexes with femtosecond pulses across the electromagnetic spectrum

“…The technique also delivers the lifetime of the metastable state through the kinetics of the ground-state bleach recovery (i.e., HS → LS transition). In few cases, transient infrared absorption (TIA) spectroscopy (Wolf et al, 2008;Zerdane et al, 2018a;Collet et al, 2019) and transient Raman spectroscopy (Smeigh et al, 2008 andCollet et al, 2019) have been used to follow the vibrational relaxation toward the dark thermalized HS state in related mononuclear Fe II complexes. Utilizing the high photon energies delivered by the X-ray Free Electron Laser (XFEL) facilities, transient x-ray emission (TXE) spectroscopies of the Fe II Kα lines (2p 3/2,1/2 → 1s) and Fe II Kβ lines (3p 3/2,1/2 → 1s) have been employed to uncover the spin multiplicity of the optically dark metal-centered (MC) states involved in the formation of the HS state (Zhang et al, 2014;Zhang and Gaffney, 2015) and, more generally, to map out the photoinduced spin-dynamics in homoleptic and heteroleptic mononuclear Fe II complexes (Zhang et al, 2017;Kjaer et al, 2018;and Kunnus et al, 2020).…”